102 STRUCTURE OF JOINTS. 



The motions permitted in joints may be referred to four heads, 

 viz.: 1. Gliding. 2. Angular movement. 3. Circumduction. 4. 

 Rotation. 



1. Gliding is the simple movement of one articular surface upon 

 another, and exists to a greater or less extent in all the joints. In 

 the least movable joints, as in the carpus and tarsus, this is the 

 only motion which is permitted. 



2. Angular movement may be performed in four different direc- 

 tions, either forwards and backwards, as in flexion and extension ; 

 or inwards and outwards, constituting adduction and abduction. 

 Flexion and extension are illustrated in the ginglymoid joint, and 

 exist in a large proportion of the joints of the body. Adduction 

 and abduction conjoined with flexion and extension, are met with 

 complete only in the most movable joints, as in the shoulder, the 

 hip, and the thumb. In the wrist and in the ankle adduction and 

 abduction are only partial. 



3. Circumduction can be performed only in the shoulder and 

 hip joints ; it consists in the slight degree of motion which takes 

 place in the head of a bone against its articular cavity, while the 

 extremity of the limb is made to describe a large circle upon a 

 plane surface. 



4. Rotation is the movement of a bone upon its own axis, and is 

 illustrated in the hip and shoulder, or better in the rotation of the 

 cup of the radius, against the rounded articular protuberance of the 

 humerus. Rotation is also observed in the movements of the atlas 

 upon the axis, in which the odontoid process serves as a pivot around 

 which the atlas turns. 



The structures entering into the composition of a joint are bone, 

 cartilage, fibro-cartilage, ligament, and synovial membrane. 



Cartilage is an elastic and apparently homogeneous substance of 

 a pearly whiteness, which forms the thin incrustation upon the 

 articular surfaces of bones. Upon convex surfaces it is thickest in 

 the centre, and thin towards the circumference, and presents upon 

 concave surfaces an opposite arrangement. It is composed of a 

 number of minute fibres placed perpendicularly to the surface, 

 attached by one extremity to the bone, and forming by the other a 

 smooth plane, covered by synovial membrane. 



Fibro-cartilage, as expressed in its name, is a compound struc- 

 ture, consisting in the combination of fibrous and cartilaginous tis- 

 sues in variable proportions. It is found in joints under three forms : 

 1. Of interafticular fibro-cartilages. 2. Of fibre-cartilages of cir- 

 cumference. 3. Of intervertebral substance. 



The inter articular Jibro-cartilages (menisci,) composed chiefly of 

 cartilage, are found in the articulations of the lower jaw, sternal 

 and acromial end of the clavicle, knee and wrist-joint. The trian- 

 gular cartilage of the wrist is not admitted by Dr. Todd* among 

 the fibro-cartilages, but is considered by him to be merely an exten- 



* Cyclopaedia of Anatomy and Physiology : article, ARTICULATION. 



